The present invention relates to a method for correcting the effect of a coexistent gas in a gas analysis, and a gas analyzing apparatus using the method.
In determining a particular gas component (measurement of an objective component or subjective component) having an absorption spectrum in the infrared region by NDIR (Non-Dispersive Infrared Detection) or FTIR (Fourier Transform Infrared Spectroscopy), there may be cases where the measurement amount (span sensitivity) is affected by the coexistent component which has likewise an absorption spectrum in the infrared region but is separated from the objective component or the coexistent component which does not have an absorption spectrum in the infrared region.
This is based on the premise that, notwithstanding the fact that there may be cases to cause differences in spectral intensity depending on the gas composition (coexistent components) with the same gas components and same gas concentrations, according to the conventional infrared absorption method, the interference between the components is attributed to the fact that a complete separation of an overlapping absorption spectra cannot be performed. In practice, it has been observed in the analysis of automobile gases that H2O and O2, which are the coexistent components to the span directives of CO and CO2 which are the objective components and do not exhibit a constant concentration, are liable to give effects.
However, since O2 does not absorb in the infrared spectrum and since it is difficult to calibrate the concentration of H2O, they are difficult to analyze under the infrared absorption method.
FIG. 6(A) shows the relations between the coexistent H2O concentration and the error of the CO2 indication value at the time when various concentrations of CO2 are measured. As the H2O concentration increases, the error of the CO2 indication value is shown largely to be positive. FIG. 6(B) shows the relation between the coexistent O2 concentration and the CO2 indication value at the time when various concentrations of CO2 are measured by using two gas analyzers. As the O2 concentration increases, the error of the CO2 indication value is shown largely to be negative. In other words, it can be seen that, because the sensitivity calibrations of these gas analyzers are carried out by a standard gas produced on the basis of N2 gas, sensitivity change has been produced in the case where the mixed gas containing H2O and O2 becomes the base gas.
Although a mechanism explaining the phenomena shown in FIGS. 6(A) and (B) is not fully understood, in one factor it can be presumed that a quenching occurs from the mutual interactions of gas molecules.
FIGS. 7A and 7B show a model where quenching may lead to a variation in the amount of infrared absorption.
The model demonstrates a supposition that the amount of variation in infrared absorption depends on the probability of collision between an objective component X and a coexistent component A along with the size of the reciprocal actions at the time of the collision. That is to say, FIG. 7(A) shows a case where both the probability of collision of the coexistent component A with the objective component X and the reciprocal actions at the time of the collision are relatively small. Because the coexistent component A has little effect upon the equilibrium of the base condition or excitation condition of the objective component X, the concentration of the coexistent component A scarcely affects the amount of infrared absorption by the component X. On the other hand, FIG. 7(B) shows the case where both the probability of collision of a coexistent component B with an objective component X and the reciprocal actions at the time of the collision are relatively large. Because the equilibrium of the objective component X is displaced to the base condition side, new light absorption becomes likely to occur. In other words, due to the presence of the coexistent component B, the absorption concentration of the objective component X becomes relatively large, and the objective component X shows a stronger absorption than the case where the base gas is of the coexistent component A and of the same concentration.
With respect to other mechanisms that can cause the above phenomenon, it is possible that a xe2x80x9ccollision spreadingxe2x80x9d mechanism broadens the width of the absorption line because the absorption wavelength is affected by the actions of the objective component itself and the coexistent component.
The phenomenon of xe2x80x9ccollision spreadingxe2x80x9d is a problematic matter even for the gas analysis of a fuel battery system which is regarded as a promising automobile power source for the next generation. FIG. 8 shows schematically a general fuel battery system 40 having a methanol supply source 41. CH3OH from the supply source 41 is supplied to a quality reformer 42, where the CH3OH is reformed under an optional catalyst to generate a reformed gas. The reformed gas contains, besides H2 gas, unreacted CH3OH, high concentrations of CO2 and H2O as bicomponents, CO, CH2 and the like as impurities. Accordingly, it is so constituted that the reformed gas, after removing the components (such as CO) which poison a fuel battery 44 at an impurity eliminating part 43, is supplied to the fuel battery 44.
Here, the concentration of the generated H2 gas, CO2, H2O, etc. falls into the range of several % to several tens %, while the concentration of CO, CH4, etc. as impurities is significantly less in the order of ppm. However, in order to favorably operate the fuel battery 44 it is desirable to reduce the concentration of impurities such as CO or the like as much as possible, which requires an accurate measurement of the concentrations. In addition, in order to verify or control the efficiency of H2 gas generation, the concentration of CH3OH or other hydrocarbons (HC) need to be measured.
Therefore, the above fuel battery system 40, has, a sample gas flow route 46 connected to the gas flow route 45 immediately following the reformer 42. The gas flow route 46 is provided with various gas analysis gauges 47 such as a Non-Dispersive Infrared Detection (NDIR) analyzer, magnetic oxygen meter, Flame Ionization Detector (FID), FTIR, etc. The fuel battery system 40 is configured such that the concentration of the impurities, such as CO, CO2, HC, etc. contained in the H2 gas generated in the reformer 42, are monitored at the output of the reformer 42 or immediately after the impurity eliminating part 43. In the case of measuring the concentration immediately after the impurity eliminating part, a sample gas flow route 49 is connected to a gas flow route 48 which is immediate after the impurity eliminating part 43. The sample gas flow route 49 is provided with a gas analyzer 50 similar to the above gas analyzer 47, and the difference of the outputs of the gas analyzers 47, 50 is taken to make it possible to grasp to what extent the above impurities have been removed in the impurity eliminating part 43. Based on the data, the reformer 42 and impurity. eliminating part 43 are controlled to supply high quality H2 gas to the fuel battery 44.
However, when measuring CO by FTIR or NDIR, for example, there is a possibility for sustaining the effect of H2 gas as the coexistent gas due to the mechanism of spectral intensity variation by the coexistent gas as described above. Furthermore, with respect to FID, the H2 gas in the sample gas is apt to affect the sensitivity in the form of a collapse of balance between the fuel gas (H2 gas) and the auxiliary combustion gas (Air) which are supplied to the detector. With respect to the O2 gauge (gas analyzer for determining the oxygen concentration), the H2 gas which is contained in only the sample gas and not in the calibration gas may affect the sensitivity.
FIG. 3 shows an effect of the H2 gas concentration in the sample gas upon the span sensitivity. Curves A, B, C, and D show variations of the span sensitivity in a CO meter, CO2 meter, THC meter (gas analyzer for measuring the whole hydrocarbon concentration), and O2 meter, respectively. For example, the span sensitivity of the CO meter, is monotonously lowered as the H2 gas concentration becomes larger. The span sensitivity for the CO2 or O2 meter monotonously increases as the H2 gas concentration becomes larger. Further, in the THC meter, the span sensitivity initially decreases with an increase in H2 gas concentration. When the H2 gas concentration passes a critical concentration, the span sensitivity increases with an increase in H2 gas concentration.
In order to accurately measure the concentration of the gases under such conditions, it becomes necessary to correct the portions which is affected by the H2 gas. Here, as the concentration of the H2 gas depends on sample gases, a correction is made in recognition of the information from the measured H2 concentration.
Conventionally, the correction is preformed by obtaining real time information on the change of concentration of the H2 gas (continuous correction). However, it is necessary to apply one or more complicated correction curves which cannot be taken as proximate to the linear state as above. Notwithstanding the very complicated correction, precision is not necessarily sufficient, and due to the difference in the response speed between the H2 gas analyzer and the other component analyzer, adequate time is required for the correction. Such differences may become a cause for errors, thus providing problems such as a loss of precision.
The present invention has been made in reflection of the situation described above. One object is to provide a method for correcting the effect of a coexistent gas during a gas analysis. The effect of a base gas, namely a coexistent gas, can be corrected simply by suppressing the affecting amount of span sensitivity caused by the difference of the base gas compositions between the calibration gas and the sample gas. A further object is to provide a gas analyzing apparatus applying the method.
In order to attain the above objects, the method for correcting the effect of the coexistent gas in the gas analysis of the first embodiment is so set that, in case of analyzing the concentration of the objective component (gas of the measuring subject) contained in the sample gas by the gas analyzer, the output of the gas analyzer is to be corrected by applying the relationship at a fixed point between the concentration of the coexistent gas and the span sensitivity of the objective component.
In order to practice concretely the above method for correcting the effect of the coexistent gas in the gas analysis, a gas analyzing apparatus is equipped with a gas analyzer for analyzing the concentration of the objective components contained in the sample gas and the operation processing part for processing the output from the gas analyzer. In the operation processing part, the output from the gas analyzer is corrected using the relationship at a fixed point between the concentration of the coexistent gas contained in the sample gas and the span sensitivity of the objective component.
Applying the relationship at a fixed position between the concentration of the coexistent gas contained in the sample gas and the span sensitivity of the objective component, the output from the gas analyzer for measuring the objective component is corrected by the same correction. Accordingly, the above output can be quickly corrected by a simple correction formula and without undergoing the effect of a difference in the response speeds between the analyzers.
The concentration of the coexistent gas contained in the sample gas may be selected from a previously known fixed amount or an amount of variation observed by actually providing a gas analyzer for detecting the concentration. Alternatively, when the change by time of the concentration of the coexistent gas is previously known, the related data may be used as such.
With respect to the method of correcting the effect of the coexistent gas in the gas analysis of a second embodiment, information, which comprises differences of the extent the base gas composition between the calibration gas and the typical actual sample along with the concentration amount of the objective component in the calibration gas is inputted into a gas analyzer at the time of the calibration of the gas analyzer. A sensitivity adjustment coefficient is determined in consideration of the effect amount which is previously stored in the gas analyzer.
According to the above method for correcting the effect of the coexistent gas in a gas analysis, it is possible to correct in advance of the span effect by the coexistent component. When the fluctuation of concentration of the coexistent component is relatively small, additional correction is not required to be made in measuring the sample gas or in processing the data, and the software becomes very simple. Furthermore, as the sensitivity adjustment is carried out simultaneously with the calibration, even in the case of measuring the measuring line having different coexistent concentrations, it suffices with re-calibration to be carried out.
In the above embodiments, it is possible to effectively correct the effects of the components which are difficult to fill in the gas cylinder (high concentration H2O and the like) and the components having variable concentration in the sample gas (H2O in engine exhaust gas and H2 gas in fuel battery system). In addition, ordinary low priced N2 balanced calibration gas may be used because a calibration gas containing such a coexistent component is not required.
Furthermore, in the first embodiment, coordinate operation is possible even when the concentration varies to a large degree by monitoring the change in concentration of the coexistent gas. In addition, in the second embodiment the errors can be suppressed to the minimum extent by setting to an average condition.